In general, desalination and purification of saline or polluted water using gas hydrates is known in the art. See, for example, the above-referenced issued patents to which this application is related, the contents of each of which are incorporated by reference and replicated below. According to those patents, a hydrate-forming gas, liquid, or mixture of gases or liquids is combined with water to be treated under pressure and temperature conditions at which hydrate spontaneously forms, i.e., conditions within the hydrate pressure/temperature stability zone for the particular hydrate-forming substance used. The hydrate is then naturally or mechanically separated from the residual brine (“brine” being used herein to refer to either residual saltwater or residual polluted water, depending on the particular context) and allowed or caused to dissociate (melt), thereby releasing fresh water (e.g., for drinking) and the hydrate-forming substance. Alternatively, as taught in co-pending application Ser. No. 10/429,765 filed May 6, 2003 (published Nov. 13, 2003 as Publication No. 2003/0209492), the contents of which are incorporated by reference, hydrate may be caused to form on or collect against a permeable support member, then be caused to dissociate against the support member (e.g., by lowering pressure on the side of the support member opposite to the hydrate), with the released hydrate-forming substance (e.g., gas) and pure water passing through the permeable support member to be collected on the opposite side.
It is also known to conduct hydrate-based desalination in an open-ocean installation, as disclosed, for example, in U.S. Pat. Nos. 5,873,262 and 3,027,320.
According to the referenced patents and published application, the hydrate may be formed in a hydrate-formation region at the bottom of a body of water (either formed in land or in an open-ocean environment), where the pressure is naturally high enough for hydrate to form. Alternatively, the hydrate may be formed in a pressurized hydrate formation vessel. In either case, the residual brine will have somewhat elevated concentrations of salt (NaCl) or other chemical pollutants, solute(s), or suspended particulate matter.
Where the primary reason for forming hydrate in seawater or polluted water is to obtain water for potable, agricultural, or industrial use, low conversion rates—i.e., relatively small percentage removal of water—are often desirable to maintain the environmental quality of the residual water, which may be returned to its source.
On the other hand, there are certain cases of naturally occurring, highly acidified bodies of water the salinities of which are in the brackish to seawater range and which are toxic to aquatic life. For certain other bodies of water, where the salinity of each of the various dissolved chemical species is much higher than naturally occurring water—the term “salinity” being used here and throughout this application in the broadest sense, as appropriate—and where the pH of the water is dramatically different from naturally occurring water, the environmental concerns associated with them and the water treatment processes typically employed with them may be substantially different from those associated with and used for treating natural seawater or brackish water. These bodies of water are not generally naturally occurring, but are commonly the residue of industrial processes and are commonly referred to as “toxic wastes.” In contrast to the case for seawater or brackish water, where total salinity and the amount of suspended matter is naturally occurring and of such compositions and concentrations as to be tolerable for a wide variety of life forms, toxic wastes are inhospitable to a wide variety of life forms and may comprise dangerous biotoxins. Therefore, in these cases, it may be desirable to reduce as much as possible the volume of water that has been made toxic by the presence of dissolved chemical substances.
In that regard, there is a general need in many industries that produce large volumes of toxic waste water to reduce the volumes of such fluids, which may be held in large ponds. For instance, the wet process manufacture of phosphoric acid as practiced in Florida and many other parts of the world requires a large volume of process water that is used as a water source for the phosphoric acid; for gas scrubbing; to slurry the phosphogypsum produced and transport it to storage; to operate barometric condensers; and for a multitude of other uses in the chemical complex. A major portion of the heat released in the process ends up in the process water and is discharged to the atmosphere by evaporative cooling. The process water is stored in holding ponds that provide the large surface area needed for evaporation and cooling of the water. (Other industries, such as the micro-electronics industry, mining, coal beneficiation, and metal coatings industries, can also produce waste water inventories that are held in such holding ponds.)
Such pond water is extremely acidic (pH=1 to 2). (For reference, most fish are killed when the water reaches a pH of 5.6, and entire lakes or other bodies of water are considered to be incapable of supporting normal aquatic life at pH 4.1). Therefore the pond water is a strong biotoxin, which if released can strongly pollute surface and ground waters, surface water including lakes, rivers, and all water that flows on the land surface and ground water being water that has sunken into the ground and resides in or moves through groundwater aquifers. Such pond water contains high dissolved salt, mineral acid, and fluorinated compounds concentrations, as identified in Table 1. The dissolved materials include ammonium, fluorosilicic acid (H2SiF6), hydrofluoric acid, fluorine, and sulfur tetrafluoride.
TABLE 1Phosphoric Acid Process Water Research Background Information.From the Florida Institute of Phosphate Research website<http://www.fipr.state.fl.us/>UntreatedProcessParameterWaterDensity (MDS)1.03–1.05Lab pH2.1Conductivity (μmhos/cm)22,100Turbidity (NTU), 24 hours—Turbidity (NTU), 72 hours—Lab Turbidity (NTU)0.9Color (Pt/Co units)300Calcium, Ca (mg/l)538Magnesium, Mg (mg/l)223Sodium, Na (mg/l)2260Potassium, K (mg/l)210Iron, Fe (mg/l)59Manganese, Mn (mg/l)15Total Chloride, Cl (mg/l)140Total Fluoride, F (mg/l)4120Sulfate, SO4 (mg/l)6200Total Phosphorus, P (mg/l)6600Ammonia Nitrogen, N (mg/l)1240Acidity, CaCO3 (mg/l)—Alkalinity, CaCO3 (mg/l)—Solids, Total Dissolved (mg/l)39,800Solids, Total Suspended (mg/l)22
In Florida, the average yearly rainfall and the pond evaporation rate are approximately equal, according to Florida Institute of Phosphate Research, and it is normally possible to operate an industrial chemical complex with a negative water balance by strict control of the water inputs to the ponds. However, in a year where rainfall is significantly above average or where water management practices fail to sufficiently reduce water inventory, it may become necessary to treat the surplus water and release it to the surface waters in order to avoid an uncontrolled discharge of the untreated process water.
Present treatment practice is to “lime” the water (i.e., add lime to it) to obtain a pH of approximately 4.5; remove the solids formed; lime the water again to a pH of approximately 11; remove the solids formed; air strip the water to remove ammonia; and add sulfuric acid to reduce the pH to approximately 6.5. Although the water will still contain dissolved solids and have a conductivity above discharge standards, under emergency situations it can be discharged to the surface waters under an emergency permit from the Florida Department of Environmental Protection. Unfortunately, the residual dissolved salts in industrial pond water present hazards to public health and the environment, and a significant release of pond water can easily destroy most aquatic organisms. Although a significant emergency release of treated water may not have such severe results, it can still cause significant biodegradation and environmental impact.
Thus, there is significant need to control total volume of such industrial pond waters.
In addition to the double-liming technique explained above, two other techniques typically are used to reduce pond water volume along with methods for adjusting the pH of the pond water (double liming).
(1) Evaporation. This is a natural method for reducing the water inventory. Evaporation is widely used in the phosphate industry. Where ponds have a very large surface area, considerable evaporation may occur. Spray techniques can be used to increase evaporation by increasing the surface area of the water. The rate of evaporation is related to the surface area of liquid exposed, the temperature of the liquid, and the relative humidity. In addition, the vapor pressure of pond water is lowered significantly due to elevated dissolved salt concentrations. The lower pond water vapor pressure lowers the rate of evaporation. Further, high humidity, as often exists in Florida, lowers the rate of evaporation; when humidity approaches 100%, little evaporation takes place at all.
Artificial evaporation desalination techniques, such as multi-stage flash techniques, have substantial associated energy costs for heating the pond water (to make the process more efficient in terms of the percentage of water that can be evaporated from a particular volume of water). In addition, because the untreated pond water may be at or close to saturation, evaporation will lead to the formation of large amounts of crystallized scale, which can clog the installation apparatus and render the process less efficient. Thus, application of artificial evaporation desalination techniques appears to be impractical, without prior treatment to lower the pH by causing substantial precipitation of dissolved and suspended matter.
(2) Conventional desalination, Reverse Osmosis (RO). This method is also being used in at least one trial installation to reduce pond water inventory. Osmosis is a natural phenomenon in which a liquid—water, in this case—passes through a semi-permeable membrane from a relatively dilute solution toward a more concentrated solution. This flow produces a measurable pressure, called osmotic pressure. If pressure is applied to the more concentrated solution, and if that pressure exceeds the osmotic pressure, water flows through the membrane from the more concentrated solution toward the dilute solution. This process, called reverse osmosis (RO), removes up to 98% of dissolved solids, and virtually 100% of colloidal and suspended matter.
The membrane must be physically strong to stand up to high osmotic pressure—in the case of seawater, about 2500 kg/m2. After filtration to remove suspended particles, incoming water is pressurized to 200-400 psi (1380-2760 kPa), which exceeds the water's osmotic pressure. As a result, a portion of the water (the permeate) diffuses through the membrane, leaving dissolved salts and other contaminants behind with the remaining water with which they are sent to drain as waste (the concentrate).
RO has a number of well-known limitations, especially in highly saline water and with dissolved solids at or near their saturation limit. A significant limitation with RO treatment of highly saline or highly chemically saturated water is membrane fouling from suspended particulates and scaling. Pond water is a saturated or nearly saturated solution of many salts. Cooling the water or raising the concentration by removing water initiates precipitation of dissolved materials, and scaling of the RO membranes will result. Therefore, RO treatment of the highly saturated pond water requires pretreatment to prevent scaling.
In addition, the little-used desalination method of freezing has been suggested for treating waste pond water. However, this method is very energy-intensive, especially if rapid freezing and subsequent melting is required, in part because the high volume of dissolved solids lowers the freezing point substantially. Also, rapid freezing causes poor rejection of dissolved solids, which can be accommodated within the hexagonal crystal structure of ice. Additionally, residual brines can be overgrown and trapped between ice crystals. Furthermore, in a hot and humid region such as Florida, pond water freezing would be prohibitively expensive.
There is thus strong need for a more versatile, cost-effective technique to decontaminate and reduce industrial pond waste water inventories. Furthermore, there are numerous other processes in which significant volumes of water need to be removed from aqueous or other water-containing products. For example, drying and concentration of food products intended for human or animal consumption is an important part of many industrial processes. Many types of food products such as orange juice, condensed milk, evaporated skim milk, powdered milk, powdered whey, amongst other food products, are concentrated by evaporation, in which considerable amounts of heat must be applied to drive off water.
In the case of dairy and other food products, heat is commonly applied under partial vacuum conditions to lower the boiling point; otherwise, the food will be spoiled, which would require its disposal, or it will have an undesirable cooked flavor. In the case of milk, the temperature must be controlled accurately. Milk is commonly heated under a vacuum that is deep enough to cause the milk to boil at between 40-45° C., and the milk is concentrated to approximately 30% solids concentration under essentially fluid conditions. This concentrated milk is either removed as a relatively viscous fluid, for packaging or other processing that does not involve further concentration, or it is further dried. Alternatively, dry powdered mild is produced through spray evaporation at 200° C. in a chamber filled with hot gas that causes the milk to form small, non-aggregated particles. The powdered milk has less than 5% water content.
Concentrated orange juice and other citrus, fruit, and vegetable concentrates are also produced by evaporation. Sometimes, but not always, the evaporation process also uses reduced pressure evaporators. However, fruit and vegetable juices are more resistant to spoiling when temperatures are raised for evaporative processes. Thus, if reduced pressure is used, the vacuum need not be so deep, and hence as expensive to produce, but the amount of heat that has to be applied is greater. Throughout the process of evaporation, the rate of evaporation slows as the material becomes more concentrated. Lowering of vapor pressure effect is a colligative property that acts broadly throughout the fluid because of the distribution of hydraulic pressure. As concentration increases, the vapor pressure of the water in the fruit or vegetable juice lowers linearly, and more heat must be applied to drive the evaporation process and remove more water. Evaporation thus becomes less efficient under conditions of increasing concentration.
Because of the requirement to apply considerable heat over short periods of time, and often under conditions where vacuum pumping must be carried out continuously, present concentration processes add significant cost to the concentrated food production process. Moreover, the efficiency of the process can decrease significantly with increasing concentration, as noted above. Accordingly, a process for concentrating aqueous or other water-containing food products that does not require such levels of heat input, and that generally remains efficient, is desirable.